Plate-shaped carbon fiber composites are used to protect equipment which would be damaged or destroyed by heat when directly exposed to a particle flux. In order to prevent this, such sensitive equipment is lined with CFC tiles, at least in the surface area that is exposed to the particle flux. Examples of such equipment include bodies entering the atmosphere, such as rocket aircraft, space gliders, or aircraft which travel at very high speeds in thin atmospheres and return to denser atmospheres. Also under consideration are vane devices which rotate at high speed in an atmosphere, in particular the outer regions of such vanes. In general, any body that is exposed to an impacting particle flux causing a high heat flux load, and which must to be protected from heat may be included here.
A further field of application of plate-shaped carbon fiber composites is that of tokamak- or stellarator-type fusion machines. Such fusion machines have diverters installed therein, which are exposed to a particle flux from the magnetically toroidally confined thermonuclear plasma. Direct exposure of such diverters to an impacting particle flux would create high thermal stress therein, which would not allow for economical operation. Because of this, the diverters, such as those used in JET and ASDEX Upgrade, are covered and mechanically reinforced with about 2 cm thick armor tiles, sized about 10×10 cm2, in the surface area that is exposed to said particle flux. The wall/armor is heated by the incident heat flux from this plasma. However, in these facilities, the controllable stationary heat flux of 10-20 MW/m2 is superimposed by short bursts of approximately 10 GW/m2, that is, a value 1000 times greater, said bursts substantially increasing the already high thermal stress, though only for very short periods of time.
NB31 and NS31 grade carbon fiber composites (CFC) were developed for the diverter armor of tokamak ITER. These carbon fiber composites have a high thermal conductivity and a low erosion rate, and are appropriate for a stationary tokamak operation at characteristic temperatures of 1000-1500 K, as described in G. Federici et al. Key ITER plasma edge and plasma-material interaction issues. Journal of Nuclear Materials Volumes 313-316, March 2003, 11-22. However, NB31 has high erosion rates due to brittle destruction in the temperature range of 3000-3500 K, which was shown by experiments using electron beam facilities and by numerical analyses of the erosion of carbon fiber composites under extreme heat flux and off-normal ITER events, such as edge localized modes (ELM) and vertical deflection events (VDE)as described in S. Pestchanyi, V. Safronov I. Landman, Estimation of carbon fibre composites as ITER divertor armour, Journal of Nuclear Materials, Vol. 329-333 (2003) 697-701 and S. Pestchanyi, H. Wuerz, Brittle destruction of carbon based materials under off-normal ITER-FEAT conditions, Phys. Scr. T91 (2001) 84-89.
Numerical simulations for NB31 using the Pegasus-3D code have revealed that the high erosion rate of NB31 is due to an erosion mechanism caused by local overheating as described in S. Pestchanyi et al, 3-D simulation of macroscopic erosion of CFC under ITER off-normal heat loads, Fusion Engineering and Design. V. 66-68 (2003) 271-276. The local overheating erosion mechanism (LOEM) results from the complex structure of CFC, which consists of a carbon matrix and a carbon fiber reinforcement. The main component of the reinforcement, the carbon or pitch fiber, has a high thermal conductivity and, in an armor of the type under discussion, is always perpendicular to the surface that is exposed to the heat flux. Such a fiber composite is woven and stitched using polyacrylnitrile fibers running parallel to the heated surface.
The large difference in the fiber and matrix coefficients of thermal expansion is decisive for the local overheating erosion mechanism. Increased erosion in the local overheating erosion mechanism is due to the preferential cracking at the interfaces between the fibers and the matrix, followed by thermal isolation of the polyacrylnitrile fibers by, or because of, the matrix. Experiments and numerical simulation both yield the same erosion pattern for the NB31 CFC. Erosion begins in the region of the polyacrylnitrile fibers, which are parallel to the surface. The erosion rate of the polyacrylnitrile fibers is always much higher than that of the highly thermally conductive pitch/carbon fibers, which are perpendicular to the surface. The “valleys” along the polyacrylnitrile fibers laterally undermine the pitch fiber regions by inducing additional erosion (see FIGS. 3 and 5), thus strongly and increasingly reducing the functional duration until catastrophe occurs, just as in a positive feedback process.
A feature of the local overheating erosion mechanism, namely the preferential erosion of the sites of woven and stitched fibers parallel to the heated surface, has been demonstrated.
The local overheating erosion mechanism of the NB31 shield/NB31 armor reveals a particularly high erosion rate for this grade of CFC. Experiments confirm an increased erosion of NB31 under repetitive loads, characteristic for type I edge localized modes in ITER.